1. Field of the Invention
The present invention relates to a method of manufacturing a compound semiconductor device, that is particularly useful for a method to avoid oxidation steps in removal of resists and cleaning the surface of the compound semiconductor device.
2. Description of the Related Art
In manufacturing a GaAs field effect transistor (GaAs FET), after forming source and drain electrodes and a recess in an active layer of GaAs, a gate metal is deposited on a photoresist mask. The photoresist mask has an opening at a central portion of the recess. Excess metal on the photoresist mask and the mask are lifted off to create a gate electrode. Then, a passivation film is deposited, completing manufacture of a GaAs field effect transistor. When the lift-off method is used, it is necessary that the residues of the photoresist be completely eliminated by ashing with an oxygen plasma or photo ozone after lifting-off the resist and excess metal. Further, in GaAs field effect transistors, trapping and releasing of electrons attributed to the surface (interface) states of GaAs crystal gives rise to a gate lag phenomenon of the transistor. Noting this, various methods such as a surface modification treatment or a plasmaless passivation have been tried to avoid influences of the surface (interface) states at the recess in the surface of the GaAs crystal (Hideaki Ikoma: Applied Physics 69, PP. 159–165 (2000)).
Contrary-to the conventional ideas, the inventor found that the gate lag is not caused by the trapping and releasing of electrons at/to the surface state as considered formerly, but is mainly due to the GaAs oxides existing on the GaAs surface.
The inventor has also found that the GaAs oxides deteriorates various characteristics of GaAs devices such as a gate/drain breakdown voltage, a FET noise or the like. Thus excellent GaAs semiconductor devices with no gate lag were developed. The novel idea was named “Asherless, i.e., a method without excess GaAs oxides”.
Further, the inventor found that resist removers containing N-methyl-2-pyrrolidone (NMP) as a component make it possible to effectively remove residues of photoresists and organic chemicals to such an extent as to omit the ashing step from the lift-off process.
However, as GaAs integrated circuits contain more field effect transistors, removal of photoresists and organic chemicals becomes more difficult and complex. Some of the residues deteriorate the entire GaAs integrated circuit and reduce the production yield of a wafer. It is recognized that the demands for improved performance of residual resist removal with avoidance of attack on GaAs field effect transistors.